Most of spare fuel tanks disposed either as a separate type, outside the main fuel tank, or as a built-in type, inside the same or integrally constructed therewith. The separate type, in some cases, communicates with the main tank, but it must then be more carefully checked for fear of being in-advertently emptied. The latter type appeared as an advanced type, and is widely used today. In one bottom corner of a main tank a certain space is separately occupied, in this type, for a spare tank by means of secluding or separating plates welded to the main tank. This advanced built-in type is apparently very convenient and practical in that it does not need individual or dual supplying of fuel because of automatic and simultaneous fuel supplying to both, and that it dispenses with the communication through piping between both tanks. However, their manufacturing cost is high. It is purpose of the current application to provide a less expensive fuel tank that notices to the driver to recharge gasoline well before the fuel tank is advertently emptied.